1. Field of the Invention
This invention relates generally to an improved dual axis translation mechanism and in particular to an x-y translation mechanism useful in an x-ray spot film device
2. Description of the Prior Art
Conventional spot film devices usually comprise a support or frame extending crosswise over the top of an x-ray table. A main carriage is mounted on the support for being advanced from a rearward parked position to a frontward radiographic position wherein a film cassette carried by the carriage is disposed in alignment with an x-ray beam that is projected through a patient from an x-ray source in the table. The film cassette is mounted in a tray supported on an inner carriage which is translatable with respect to the main carriage so that the center of the collimated x-ray beam may be made coincident with an area on the film cassette where one or a sequence of spot film exposures are to be made. The area is further defined by superimposable x-ray masks.
As is well known, spot film devices are also used in conjunction with a fluoroscopic device which permits an examining radiologist to visualize anatomy of interest before making one or more radiographs in a choice of sizes by translating the film cassette forward and shifting it, and the masks, to obtain the desired sequence of radiographs. The fluoroscopic device on the spot film device is aligned with the x-ray source in the table, and the film cassette is, of course, retracted from the x-ray beam during fluoroscopy.
When a fluoroscopic view of interest is observed, the film cassette must be projected into the x-ray beam path rapidly and one or more exposures must be taken while the fluoroscopically observed condition persists.
The x-y translating mechanism of spot film devices generally employ electromechanical means for rapidly advancing and retracting the film cassette between load and parked positions and between parked and the various positions in which the sequence of radiographs are taken. In addition, means are provided for predetermining the sequence and for cushioning the shock forces that are incidental to rapid transfer of the film cassette carriage from the parked position to its other positions when alternating between fluoroscopic and radiographic or loading modes.
These functions have been achieved in known translation mechanisms by complicated arrangements of mechanical linkages, tracks, cams, relays, belts and so forth, which accomplished their purposes under manual or mechanical influence or under a combination of such influences. Cassette translation mechanisms including motor drive means typically include two reversable motors, one motor for each of the orthogonal directions in which the film cassette must be driven. The requirement of two motors undesirably increases the weight and power consumption of the drive system. In other types of motor systems having one and/or two motors, as a motor moves the film cassette rearwardly to a parked position, it slowly loads a return spring at the same time. The film carriage is latched in the parked position and when the latch is released, the carriage is advanced rapidly under the influence of the spring and halted abruptly in the radiographic position. Rapid movement and abrupt halt of the film carriage results in considerable noise, shock and vibration that necessitates use of shock absorbing devices such as dash pots to reduce these ill effects. One problem with this type of system is that the main carriage must be restored to a rearward position after each exposure, to reload the spring, after which the carriage must be projected forwardly again to make the next exposure.
Other types of prior spot film devices have a set of tracks for the main carriage After each exposure, the carriage is returned rearwardly and shifted to different tracks, similar to railroad car switching. When the carriage is driven forwardly, it arrives in the proper position for the next exposure to be made. This is a relatively slow method and requires a large and complicated mechanism which has many moving parts.
The above arrangements have resulted in lower than desirable reliability, increased power consumption, a massiveness that has had to be off-set with increased counterweight and/or power and a noisy operation. Some of the complexity and size resulted from spot devices being adapted to accommodate rectangular cassettes in their long and short dimensions in which case means had to be provided for altering the mode of operation of the transfer and sequence mechanism depending on how the cassette was oriented in its holder.
The present invention is directed to overcoming the abovenoted disadvantages. It is a general object of this invention to provide a translation mechanism particularly useful in a spot film device that is simple in construction, lightweight, operates quietly, efficiently, safely, and automatically, is simple to operate and maintain and is comparatively inexpensive to manufacture.